Optical sources are used in a wide range of applications including telecommunications, optical sensing and reading/writing optical device media. Currently, the majority of optical devices are designed on the principles of classical optics, such as physical optics and geometrical optics. Thus, physical and geometrical optics are typically used when modelling the propagation behaviour of light. When light interacts with a physical material, for example by being absorbed, a quantum mechanical point of view of light being quantized photons can be used successfully to model and predict the behaviours of the photons.
The quantum theory of light however also predicts other light behaviours. One such behaviour is quantum entanglement which has no counterpart in classical optics. Quantum entanglement typically occurs when photons, commonly a pair of photons, interact and are together described by the same indefinite quantum state when physically separated. Once separated, the photons are correlated such that when a measurement is made on one of the photons, for example measuring the polarisation as vertically polarised, the other entangled photon immediately takes the appropriate correlated or anti correlated value, for example being horizontally polarised. Photons may be entangled in a variety of degrees of freedom including, but not limited to spatial entanglement and polarisation entanglement.
There are several methods of creating entangled photons. Typical methods use correlated photons created from the nonlinear elastic χ(2) or χ(3) susceptibilities of a medium to an input electromagnetic field where one or more input photons (often termed pump photons) get converted to two or more new correlated photons (often termed signal and idler photons). The signal and idler photons generated by the same nonlinear event are correlated because both are always generated during the event.
An example of creating two correlated photons using the χ(2) susceptibility is parametric down conversion (PDC) or spontaneous parametric down conversion (SPDC) where the probability of converting an input photon to a signal and idler photon pair is linearly proportional to the intensity of the input pump.
An example of creating two correlated photons using the χ(3) susceptibility is spontaneous four wave mixing (SFWM) where the probability of converting an input pump photon to a correlated photon pair is quadratically proportional to the intensity of the input pump because two pump photons are required to produce the two new photons.
One application that uses quantum entanglement is quantum cryptography. Quantum cryptography can be implemented using single photons or a pair of polarisation entangled photons. When using the latter technique, polarisation entangled photons are created using a non-linear process such as SPDC whereby pump photons are converted to entangled signal and idler photons. The converted signal and idler photons have the same polarisation but have different wavelengths. The signal and idler photons may be converted to the same polarisation as the pump photon or the orthogonal polarisation, whereby the efficiency for converting to an orthogonal polarisation is typically lower than converting to the same polarisation. In operation, a source of polarisation entangled photons sends idler photons of both horizontal and vertical polarisations to one user and sends the corresponding polarisation entangled signal photons to another user. The two users create quantum keys from the entangled photons.
US2009/0103736 describes a device for generating polarisation-entangled photons using an integrated optical waveguide device. Pump photons are split into two waveguide arms using an integrated optical splitter. Each arm has a Periodically Poled Lithium Niobate (PPLN) structure to down convert the pump photons. The PPLN waveguide arms [are not identical because they] are configured differently to one another so that one arm generates TE polarisation signal and idler photons whilst the other arm generates TM polarisation signal and idler photons. The optical modes output from the two arms are therefore distinguishable from one another. The arms are then recombined and the signal/idler photons are split using a polarisation insensitive wavelength separating device.
WO2008/039261 describes a device for generating polarisation-entangled photons using an integrated optical waveguide device that is similar to US2009/0103736. The arms in WO2008/039261 share a common down conversion device/structure, however one arm further contains a mode converter to orthogonally convert the polarisation state of the polarisation entangled signal/idler photons along that arm such that when the arms are recombined the polarisation entangled signal/idler photons from one arm are in a TE polarisation whilst the polarisation entangled signal/idler photons from the other arm are in a TM polarisation. The waveguide arms in WO2008/039261 are different. The optical modes output from the two arms are therefore distinguishable from one another.
Sources of single photons are useful for a variety of applications other than quantum cryptography such as quantum metrology and quantum computing. Proof-of-principle demonstrations for the generation, detection and manipulation of photons at the single-photon level have been shown however, these demonstrations usually involve large-scale optical elements and rely on un-scalable bulk single-photon sources and detectors. The following publications describe some existing set-ups for generating single photons.
“Telecommunications-band heralded single photons from a silicon nanophotonic chip”; Applied Physics Letters 100, 261104, 2012, Davanco et al; describes a silicon nanophotonic device made up of an array of coupled rings on a silicon photonic integrated chip.
“Evidence for phase memory in two-photon down conversion through entanglement with the vacuum”; Physical review A, vol. 41, no. 1, 1 Jan. 1990, Z. Y. Ou et al, describes a bulk optic experiment. FIG. 2 of this publication shows a pump beam split by a beam-splitter into two LiIO3 sources, each configured to produce signal and idler photons along physically different output optical paths. The idler output paths of each source recombine at a first recombining beam splitter, whilst the signal output paths of each source recombine at a different second recombining beam splitter. The signal and idler photons generated by the same source do not occupy the same mode in the same physical channel.
“Continuous wave photon pair generation in silicon-on-insulator waveguides and ring resonators” Optics Express, Vol. 17 No. 19, 16559, 14 Sep. 2009 describes producing a distinguishable pair of photons in a single silicon waveguide.